Method for producing multilayer stretch-molded article

ABSTRACT

A transparent multilayer stretched product is produced through a process including: providing a resin laminate including at least on layer of polyglycolic acid resin, heat-forming and cooling the resin laminate, reheating the resin laminate until the polyglycolic acid resin is crystallized to be opaque and then stretching the reheated resin laminate. The thus-obtained multilayer stretched product is excellent in gas-barrier property in addition to the transparency and is suitable as a packaging material or container.

TECHNICAL FIELD

The present invention relates to a transparent multilayer stretchedproduct suitable for use as a packaging material or container having agas-barrier property. The resultant transparent multilayer stretchedproduct is useful as a packaging material or container for foods, drugs,etc.

BACKGROUND ART

Polyester-type polymers, particularly polyethylene terephthalate (i.e.,terephthalic acid-ethylene glycol polycondensation product) generallycalled a “PET resin” is widely used for providing a packaging materialfor beverages having a shape of typically a bottle, that is so-called“PET bottle”, because of its transparency, rigidity, easiness offorming, etc.

The scope of applied uses is being enlarged year by year, and someapplication requires the minimization of oxygen or carbon dioxidetransmission therethrough depending on the content materials, so that itis desired to improve the gas-barrier property of PET resin.

As a means for improving the gas-barrier property of PET resin, amultilayer product of PET and polyglycolic acid resin has been disclosed(U.S. Pat. No. 4,424,242 to Barbee), but in the Examples thereof, thefilms of these resins were pressed together at 210° C., so that becauseof poor adhesion therebetween, these films are presumed to be liable topeel from each other and be accompanied with difficulty, such as poorappearance and difficulty in maintenance of the performances.

For providing PET bottles, PET is frequently formed by the so-called hotparison scheme wherein PET is stretched and blown within a period priorto crystallization of PET, or the so-called cold parison scheme whereinafter being injection-molded, PET is quenched to form an amorphousproduct (called a preform), which is reheated to above Tg, stretched andblown.

Recently, there has been studied a trial of forming a multilayer productof polyglycolic acid resin and such a PET resin and stretch-blowing itto form a multilayer bottle. However, as polyglycolic acid resin hashigh crystallinity, it causes whitening due to crystallization beforestretch-blowing either according to the hot parison scheme or accordingto the cold parison scheme. When such a crystallized polyglycolic acidresin is forcibly stretched, it is liable to cause puncture orbreakable, or stretching irregularity. For these difficulties, it hasbeen believed impossible at all to form a multilayer stretched product,such as gas-barrier bottle, including polyglycolic acid resin.

DISCLOSURE OF INVENTION

The present invention aims at providing a transparent multilayerstretched product suitable as a gas-barrier packaging material orcontainer including a polyglycolic acid resin layer.

More specifically, the present invention provides a process forproducing a transparent multilayer stretched product, comprising:providing a resin laminate including at least one layer of polyglycolicacid resin, heat-forming and cooling the resin laminate, reheating thelaminate until the polyglycolic acid resin layer is crystallized to beopaque, and then stretching the re-heated resin laminate.

Some description of a history through which the present inventors havemade various studies with above-mentioned object and arrived at thepresent invention, will be briefly made.

The present inventors, et al, have proposed a multilayer stretchedproduct based on a finding that a laminate obtained by using a copolymerof glycolic acid as a polyglycolic acid resin and laminating it withanother thermoplastic resin, such as PET, can be stretched at atemperature exceeding Tg while alleviating crystallization accompaniedwith whitening (WO-A 03/099562). However, the use of polyglycolic acidresin in the form of a copolymer adversely affects the gas-barrierproperty which is a characteristic thereof, so that the use of a higherglycolic acid content polymer is desired even if a copolymer is used,and more preferably the use of glycolic acid homopolymer is desired, ifat all possible, for providing a multilayer stretched product suitableas a gas-barrier packaging material or container.

As a result of further study in view of the above, the present inventorshave found that in the case of using a polyglycolic acid resin having avery high polyglycolic acid content and a high gas-barrier property,when a resin laminate including a layer of such a polyglycolic acidresin is allowed to cause crystallization of the polyglycolic acid resinto such a degree as to cause a whitening up to a haze of 40% or higherobstructing seeing therethrough unlike the conventional case of forciblysuppressing the crystallization of polyglycolic acid resin, a stretchingcan be rather smoothly accomplished to provide a multilayer stretchedproduct clarified to exhibit a haze of 10% or below if the stretching isperformed after uniformly heating and crystallizing the laminate to atemperature exceeding the crystallization temperature. As a result offurther study thereafter, it is considered that the above phenomenon isattributable to occurrence of so-called “crystal stretching” which hasbeen studied with respect to another resin (e.g. “Crystal Stretching ofPolyethylene Terephthalate”, Journal of The Society of Fiber Science andTechnology, Japan, Vol. 21, No. 103 (1965), pp. 528-535). Morespecifically, it is presumed that polyglycolic acid molecular chaincrystallized to such a degree as to cause noticeable whitening isre-arranged due to crystal stretching to be clarified, whereas it hasnot been known heretofore and was a really unexpected discovery thatpolyglycolic acid resin has capability of crystal stretching accompaniedwith remarkable clarification. The present invention is based on theabove finding.

BEST MODE FOR PRACTICING THE INVENTION

Hereinbelow, the process for producing a multilayer stretched productaccording to the present invention will be described more specificallyin order with respect to preferred embodiments thereof.

(Polyglycolic Acid Resin)

A principal component layer of the multilayer stretched product producedaccording to the present invention comprises a polyglycolic acid resin(hereinafter sometimes referred to as a “PGA resin”). The PGA resin mayinclude homopolymer or copolymer including glycolic acid-recurring unitrepresented by formula (I) below:—(—O—CH₂—CO—)—  (I).

The above glycolic acid unit can also be provided by polycondensation ofglycolic acid, glycolic acid alkyl ester or glycolic acid salt but maypreferably be provided by ring-opening polymerization of glycolide (GL)that is a bimolecular cyclic ester of glycolic acid.

In the multilayer stretched product of the present invention, thepolyglycolic acid resin layer may preferably be incorporated as agas-barrier resins layer. It is preferred that the polyglycolic acidresin layer functions as an effective gas-barrier resin layer even whenincluded as a layer occupying 10 wt % or less in a resin laminate withanother thermoplastic resin. In order to form such an excellentgas-barrier resin, it is possible to use a copolymer (PGA copolymer)containing at least 80 wt %, preferably at least 85 wt %, particularlypreferably at least 90 wt %, most preferably 95 wt %, of polyglycolicacid (PGA) polymerized units, whereas homopolymer (PGA homopolymer)should be selected in order to obtain the highest level of gas-barrierproperty. PGA resin having such a high PGA polymerized unit contentnaturally has a high crystallinity and can be very advantageously usedin the process for producing a multilayer stretched product according tothe present invention relying on crystal stretching as an essentialfactor.

Examples of comonomer providing PGA copolymer together with a glycolicacid monomer, such as the above-mentioned glycolide, may include: cyclicmonomers, such as ethylene oxalate (i.e., 1,4-dioxane-2,3-dione),lactides, lactones (e.g., β-propiolactone, β-butyrolactone,pivalolactone, γ-butyrolactone, δ-valerolactone,β-methyl-δ-valerolactone, and ε-caprolactone), carbonates (e.g.,trimethylene carbonate), ethers (e.g., 1,3-dioxane), either esters(e.g., dioxanone), amides (ε-caprolactam); hydroxycarboxylic acids, suchas lactic acid, 3-hydroxypropanoic acid, 3-hydroxybutanoic acid,4-hydroxybutanoic acid and 6-hydroxycaproic acid, and alkyl estersthereof; substantially equi-molar mixtures of aliphatic diols, such asethylene glycol and 1,4-butanediol, with aliphatic dicarboxylic acids,such as succinic acid and adipic acid, or alkyl esters thereof; andcombinations of two or more species of the above. Among these, it ispreferred to use a comonomer selected from the group consisting oflactide (LA; including optical isomers, such as L-lactide (LLA),D-lactide and DL-lactide (DLLA)), trimethylene carbonate andcaprolactone (CL).

The PGA resin may preferably have a weight-average molecular weight(based on polymethyl methacrylate) in a range of 50,000-800,000according to GPC measurement using hexafluoroisopropanol solvent. If theweight-average molecular weight is too low, it can have only weakstrength and is liable to craze or crack at the time of stretching, etc.If the weight-average molecular weight is too large, the resin layerthicknesses are liable to be non-uniform at the time of laminateforming, thus failing to provide a good stretched product, and the PGAresin is liable to generate heat due to shearing force exerted by screwat the time of melt-processing, cause coloring of the resin at the timeof processing to form pellets or forming into the product and cause poorappearance by occurrence of irregularity (or flow marks) in the productdue to poor melting. A weight-average molecular weight of ca.120,000-300,000 is further preferred.

As preferred thermal properties for smoothly practicing the crystalstretching in the present invention, the PGA resin may preferably haveTg (glass transition temperature) of 30-55° C., more preferably 35-50°C.; Tc1 (crystallization temperature in the course of temperatureincrease) of 60-135° C., more preferably 65-120° C.; Tc2(crystallization temperature in the course of temperature decrease) of140-200° C., more preferably 145-195° C.; Tm (melting point) of 150-230°C., more preferably 180-225° C.

In the polyglycolic acid resin layer, it is possible to incorporateanother thermoplastic resin in addition to the above-mentioned PGAresin, but even in that case, it is preferred that the polymerized PGAunits should occupy as high a percentage as possible and at least 80 wt% of the resin constituting the polyglycolic acid resin layer. It isfurther preferred that the polyglycolic acid resin layer is composed ofthe PGA resin alone (though possibly containing additives such as athermal stabilizer therefor) to effectively proceed with its crystalstretching.

To 100 wt. parts of the PGA resin, it is possible to add 0.003-3 wt.parts, more preferably 0.005-1 wt. part, of a thermal stabilizer. Thethermal stabilizer may be selected from compounds functioning asanti-oxidants for polymers, and among those, it is preferred to use,e.g., heavy metal-deactivating agents, phosphoric acid esters includinga pentaerythrithol skeleton (or a cyclic neopentane-tetra-il structure)and represented by formula (II) below, phosphor compounds having atleast one hydroxyl group and at least one long-chain alkyl ester groupand represented by formula (III) below, and metal carbonate salts. Thesecompounds can be used singly or in combination of two or more species.

(Another Thermoplastic Resin Layer)

According to the present invention, a resin laminate is formed bydisposing another thermoplastic resin layer in lamination with theabove-mentioned polyglycolic acid resin layer.

As such another thermoplastic resin, it is possible to use an arbitrarythermoplastic resin which can be laminated with the PGA resin layer asby extrusion lamination, dry lamination or wet lamination; coating; orco-extrusion or co-injection with PGA resin.

More specifically, preferred examples of such another thermoplasticresin may include: polyester resins, such as polyethylene terephthalateand polyethylene naphthalate, polystyrene resins, acrylic acid ormethacrylic acid resins, nylon resins, sulfide resins such aspolyphenylene sulfide, and polycarbonate resins. Among these, it ispreferred to use a polyester resin, particularly an aromatic polyesterresin composed of a diol component and a dicarboxylic acid component, ofwhich at least one, particularly the dicarboxylic acid component, is anaromatic one, in order to provide a multilayer product which satisfiestransparency and gas-barrier property in combination depending on theuse thereof.

Another preferred class of such another thermoplastic resin may includeanother biodegradable resin capable of providing a multilayer stretchedproduct having a high biodegradability as a whole together with the PGAresin layer. Examples thereof may include: other aliphatic polyesters,such as polylactic acid, succinic acid-glycol polycondensate andpolycaprolactone, and partially aromatic polyesters (e.g., “BIOMAX”,made by Du Pont and Co., a succinic acid-based partially aromaticpolyester).

(Resin Laminate)

The content of the polyglycolic acid resin layer in the multilayerstretched product, i.e., ordinarily in the resin laminate subjected tostretching, may preferably 1-10% on the basis of weight (nearly equal tothe percentage based on thickness). In excess of 10 wt %, the PGA resinlayer is liable to cause excessive crystallization at the time ofheating for stretching of the resin laminate so that a considerablylayer stress is required for the stretching, and such a thickpolyglycolic acid resin layer tends to be less clarified even by thecrystal stretching. Below 1 wt %, the gas-barrier property of theresultant multilayer stretched product is liable to be insufficient, andit becomes difficult to achieve the object of the present invention toprovide a multilayer stretched product having good gas-barrier propertyin combination with transparency.

In the process of the present invention, the above-mentioned resinlaminate of polyglycolic acid resin and another thermoplastic resinformed by co-extrusion, co-injection, extrusion lamination, etc., isonce cooled.

In the case where the multilayer stretched product is in the form of abottle, a so-called preform is taken out of a mold, etc., after thecooling. The PGA resin in this stage is amorphous and transparent. Anamorphous preform ordinarily has a thickness of ca. 2-10 mm and exhibitsa haze of ordinarily below 40% at a thickness of 3 mm while tending toexhibit a higher haze at a larger thickness.

In the case of producing a film or sheet product, the resin laminate isobtained as a film or sheet which has been melt-processed and extrudedout of an extruder. The film or sheet is ordinarily cooled on a rollermold, etc. and the PGA resin is taken out in an amorphous andtransparent state. Such a film or sheet a thickness of ordinarily ca. 30μm to 3 mm and exhibits a haze of ordinarily below 10% at a thickness of200 μm while tending to exhibit a higher haze at a higher thickness.

(Heating Prior to Stretching of Resin Laminate)

The resin laminates having hazes exceeding the above values in therespective forms after the cooling cannot be stretched in most casesbecause of large spherulites formed during the cooling crystallization.Even if stretching becomes possible by heating thereafter, uniformstretching is difficult.

Then, according to the present invention, the resin laminate having ahaze of below 40% in any case is subjected to heating accompanied withcrystallization as to provide a haze of at least 40%, whereby atransparent form product is obtained through a stretching subsequentthereto. While the mechanism has not been fully clarified as yet, it isconsidered that as a result of the heating accompanied withcrystallization up to a haze of at least 40%, the polyglycolic acidresin forms a uniform crystal state allowing a stretching free fromirregularity.

If it is tried to suppress the haze after the heating below 40%, thestretching condition is restricted and it becomes difficult to obtainuniform and transparent products in many cases, and a severe restrictionis posed to the selection of the resins forming the multilayer product.

It is preferred that the haze is increased by at least 5%, morepreferably by at least 10%, particularly preferably 20% or more, priorto the stretching.

The heating for the crystallization prior to stretching may be performedby appropriately adopting a method of infrared ray heating, hot airheating, electromagnetic heating, heating with a heating medium. Theheating temperature may preferably be at least Tc1 (crystallizationtemperature in the course of temperature increase), more preferably Tc1+at least 1° C., further preferably Tc1+ at least 5° C. and below Tm(melting point) of PGA resin. More specifically, a range of 80-200° C.,particularly 90-150° C., is preferably adopted. The heating step and thesubsequent stretching can be successive or non-successive.

(Stretching)

An opaque resin laminate having an increased haze of at least 40%,particularly at least 50% in the case of using PGA homopolymer,resultant after the above heating and crystallization, is thereaftersubjected to a various shaping method accompanied with stretchingcorresponding to a shape of the objective multilayer stretched product.As mentioned above, the multilayer stretched product according to thepresent invention including a PGA resin layer and another thermoplasticresin layer may assume various forms, such as films, sheets, extrusionproducts and hollow shaped products. The films may preferably assume aform of a stretched film or a heat-shrinkable film. The sheets may befurther formed into containers or vessels, such as trays and cups, bysheet-forming technique, such as vacuum forming or pressure forming. Thehollow shaped products may include blown containers, andstretched-and-blown containers. Further, it is also possible to apply aninflation technique. In the course of such a shaping process, the resinlaminate is stretched. The stretching may be either uniaxial or biaxial(simultaneous or successive). The preferable degree of stretching canvary depending on the use of the shaped product but may preferably atleast 2 times, particularly ca. 4-25 times, in terms of an areal ratio,from the view points of increased strength, improved gas-barrierproperty, improved moisture resistance, etc.

Due to the effect of crystal stretching, the multilayer stretchedproduct after the stretching is provided with a haze of at most 10%decreased from the haze of at least 40% before the stretching,regardless of its thickness.

(Post Treatment)

After the above stretching-shaping, it is possible to add a posttreatment, such as heat-setting, or a post step, such as a laminationprocessing or coating for providing an additional resin layer. Thetreatment temperature for heat-fixation may preferably be ca. 40-210°C., and at a temperature below the melting point of the PGA resin, morepreferably in a temperature range of 10-70° C. below the melting pointof PGA resin.

The lamination processing may include wet lamination, dry lamination,extrusion lamination, hot melt lamination, and non-solvent lamination.

(Gas-Barrier Multilayer Stretched Product)

The gas-barrier and transparent multilayer stretched product obtainedaccording to the present invention is suitably used as a bottle asrepresented a PET bottle or a packaging film like a wrapping film or formeat packaging, etc. Further, if a principal resin layer of themultilayer stretched product is composed of a biodegradable resinsimilar to PGA resin, a biodegradable and gas-barrier multilayerstretched product can be provided.

EXAMPLES

Hereinbelow, the present invention will be described more specificallybased on Examples and Comparative Examples.

The physical properties described herein including those described beloware based on values measured according to the following methods.

(1) Haze (%)

Measured by using “HAZE METER TCH-III-DP” made by Tokyo Denshoku K. K. Apreform sample was split vertically, and the curved concave surface wasexposed to incident light for the measurement. As for a bottom sample, aflat surface body part was cut out, and the inner surface thereof wasexposed to incident light for the measurement.

(2) Parison Surface Temperature

Measured by using a non-contact thermometer (“IT2-50”, made by K. K.Keyence).

(3) Oxygen Permeability

Measured by using “OX-TRAN 2-20” made by Mocon Co. As for a bottlesample, an adapter was attached to the mouth thereof for measurement ata temperature of 23° C., a bottle inside humidity of 80% RH and a bottleoutside humidity of 50% RH. As for a film sample, the measurement wasperformed at a temperature of 23° C. and a humidity of 80% RH.

(4) Thermal Properties

A differential scanning calorimeter (made by Metter Instrumente A.G.)was used for measurement of the following heat capacity transition Ts.

Tg: Secondary transitions temperature on a calorie curve in the courseof temperature increase

Tc1: Exothermic peak top temperature due to crystallization in thecourse of temperature increase

Tm: Endothermic peak top temperature due to crystal melting in therecourse of temperature increase

Tc2: Exothermic peak top temperature due to crystallization in thecourse of temperature decrease

(5) Melt Viscosity

Measured by using “CAPILOGRAPH 1-C” made by Toyo Seiki K. K. equippedwith a capillary (1 mm-diameter×10 mm-length) at a temperature of 270°C. and a shear rate of 122 sec⁻¹.

(6) Weight-Average Molecular Weight

Measured by using a GPC apparatus (“Shodex-104, made by Showa DenkoK.K.) with two columns of “HF1P606M” (made by Showa Denko K.K.) andusing a 5 mM-solution of sodium trifluoroacetate inhexafluoroisopropanol as the elution liquid and an RI (refractive index)detector. Molecular calibration was performed with polymethacrylic acidstandard molecular weight samples.

Examples 1 and 2

PET (“Grade 9921,” IV=0.8 (by catalogue), made by Eastman Kodak Co.) andPGA homopolymer A (Tc1=90° C., Tm=221° C., Melt viscosity=920 Pa.s,weight-average molecular weight=220,000) were respectively injected(melt-processed) by means of an injection molding machine equipped withtwo cylinders to form U-shaped parisons (precursors for formation of abottle by stretch-blow molding having a vertical sectional shape ofcharacter “U”, having a total weight of 28 g, a thickness of 3.7 mm atthe body) wherein the PGA homopolymer occupied 8 wt %.

The U-shaped parisons were subjected to stretch-blow molding at a rateof 900 BPH (bottles per hour) by using a stretch-blow molding apparatus(“SBO-1”, made by SIDEL Co.) under the conditions of re-heating to asurface temperature of 97° C. (Example 1) or 105° C. (Example 2) in atemperature-rising time of 20 sec. and a holding time of 20 sec. bymeans of an IR (infrared) heating apparatus, immediately followed bystretch-blow molding into a mold at 5° C. with compressed air, to obtaintransparent bottles of 500 ml in volume and a thickness of 0.4 mm at thebody. The hazes of injected and cooled U-shaped parisons, re-heatedU-shaped parisons and bottles after the blow molding are shown in Table1 below. Incidentally, the hazes of the re-heated U-shaped parisons weremeasured by quenching the U-shaped parisons just after discharge out ofthe IR heating apparatus (formed in parallel with those subjected tostretch blow molding) with liquid nitrogen and measuring the quenchedparisons. The surface temperatures of the U-shaped parisons immediatelyafter discharge out of two IR apparatus were measured by a non-contacttype thermometer.

TABLE 1 Bottle U-shaped parison Oxygen Haze Reheated Haze perme- aftersurface after ability Exam- cooling temp. re-heating Haze (cc/ ple (%)(° C.) (%) (%) bottle/day) 1 38 97 84 3 0.05 2 38 105 83 0.5 0.05

It is noted that the hazes exceeding 80% of the parisons after there-heating were abruptly lowered to 3-0.5% after the stretch-blowmolding to have provided transparent bottles with good gas-barrierproperty.

Example 3

U-shaped parisons (having a haze of 38%) were prepared by injection andcooling in the same manner as in Example 1, and then heated at 50° C.for 24 hours for crystallization to provide the parisons with a haze of57%. The U-shaped parisons were re-heated to 97° C. in atemperature-rising time of 20 sec. and holding time of 20 sec. in thesame manner as in Example 1, followed by injection molding to obtainbottles. The results are summarized in Table 2

TABLE 2 U-shaped parison Bottle Haze after Oxygen Reheated heatingperme- surface at 50° C. Haze after ability Haze after temp. for 24 hrsre-heating Haze (cc/ cooling (%) (° C.) (%) (%) (%) bottle/day) 38 97 5784 1 0.05

Example 4

Bottles were prepared by formation of U-shaped parisons, re-heating andblow molding in the same manner as in Example 1 except for using aPGA/PLA (95/5 by weight) copolymer (Tc1=100° C., Tm=209° C., meltviscosity=640 Pa.s, weight-average molecular weight=180,000) instead ofPGA homopolymer and re-heating to a temperature of 115° C. in atemperature-rising time of 35 sec. and a holding time of 20 sec. Theresults are summarized in Table 3.

TABLE 3 U-shaped parison Bottle Haze Reheated Oxygen after surface Hazeafter permeability cooling(%) temp.(° C.) re-heating(%) Haze(%)(cc/bottle/day) 38 115 81 1 0.09

Example 5

Bottles were prepared by formation of U-shaped parisons, re-heating andblow molding in the same manner as in Example 1 except for using anotherPGA homopolymer (Tc1=93° C., Tm=220° C., melt viscosity=1150 Pa.s,weight-average molecular weight=240,000) instead of PGA homopolymer Aused in Example 1. The results are summarized in Table 4.

TABLE 4 U-shaped parison Bottle Haze Reheated Oxygen after surface Hazeafter permeability cooling(%) temp.(° C.) re-heating(%) Haze(%)(cc/bottle/day) 38 95 84 1 0.05

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, there isprovided a transparent multilayer stretched product including apolyglycolic acid resin layer and suitable as a gas-barrier packagingmaterial or container by effectively utilizing crystal stretching ofpolyglycolic acid resin.

1. A process for producing a transparent multilayer stretched product,comprising: providing a resin laminate including at least one layer ofpolyglycolic acid resin, heat-forming and cooling the resin laminate,reheating the laminate to a temperature of Tc1 (crystallizationtemperature in the course of temperature increase)+ at least 1° C. untilthe polyglycolic acid resin layer is crystallized to be opaque, asrepresented by a haze of at least 40%, and then stretching the re-heatedresin laminate to provide a stretched resin laminate with a reduced hazeof at most 10%.
 2. A process according to claim 1, wherein the resinlaminate after the cooling is transparent.
 3. A process according toclaim 1, wherein the polyglycolic acid resin occupies at most 10 wt. %of the resin laminate.
 4. A process according to claim 1, wherein thepolyglycolic acid resin layer comprises a polyglycolic acid resin havinga sufficiently high content of polymerized glycolic acid units as toexhibit a gas-barrier property.
 5. A process according to claim 4,wherein the polyglycolic acid resin comprises glycolic acid homopolymer.6. A process according to claim 1, wherein the resin laminate includesan aromatic polyester resin layer in addition to the polyglycolic acidresin layer.
 7. A process according to claim 1, wherein the resinlaminate includes another biodegradable resin layer in addition to thepolyglycolic acid resin layer.